In an electrophotographic process, an electrostatic latent image formed on a photoreceptor is developed with a toner containing a colorant, and the toner image formed is transferred onto a transfer paper and fixed by a heat roll, etc., to obtain an image. The photoreceptor after using is subjected to cleaning for forming an electrostatic latent image again. A dry type developer used for such an electrophotographic process, etc., is generally classified into a one-component developer using a toner itself formed by dispersing a colorant in a binder resin and a two-component developer made of a mixture of the toner and a carrier. Also, in the case of carrying out the image formation using such a developer, in order that the developer has a process adaptability, it is necessary that the developer is excellent in the fluidity, conveying property, fixing property, charging property, transferring property, and cleaning property. Also, recently, the requirement of color-image formation, and particularly, on-demand printing is high, and to cope with high-speed many-copy copying, a heat roller system has been widely used for a fixing step because a high-speed fixing is possible.
As a problem in the fixing system, because a toner image in a molten state is brought into contact with the surface of a heat roller under pressure, a part of the toner image sticks to the heat roller, the stuck toner is transferred to sometimes cause a so-called offset phenomenon of staining the copied images. For the purpose of preventing the occurrence of the phenomenon, a method of coating the surface of the heat roller with a silicone rubber or a fluorine resin excellent in a releasing property and further supplying a liquid having a releasing property, such as a silicone oil, etc., to the surface thereof has been employed. The method is very effective in the point of preventing the occurrence of the offset phenomenon of a toner but in the method, there is a problem that an apparatus for supplying an offset-preventing liquid becomes necessary. This is contrary to small-sizing and light-weighing of the copying machine, and also there occurs a problem that the offset-preventing liquid is evaporated by heating to give an unpleasant smell and sometimes stain the inside of the copying machine.
To solve these problems, a method of limiting the viscosity of the toner (Japanese Patent Laid-Open Nos. H1-133065, H2-161466, H2-100059, and H3-229265), a method of incorporating a wax such as a resin having a releasing property in a toner (Japanese Patent Publication No. S52-3304), a method of limiting the melt viscosity of a wax (Japanese Patent Laid-Open Nos. H3-260659 and H3-122660), etc., are proposed.
However, because a release agent is inferior in the light transmittance as compared with a binder resin, in the case of color toners, on considering the reproducibility of OHP transparent images, a large amount of the release agent as in the case of a black and white toner cannot be added.
With regard to fixing, a wax exhibits the function by dissolving out from a toner and appears on the interface between a fixing roll and the toner. However, only a part of the wax dispersed in a toner is dissolved out from the toner and it is better that the wax exists on the surface of the toner or a release agent having a low melt viscosity is used so that at fixing, the release agent instantly moves to the interface between the toner and a fixing roll. However, these conditions become disadvantageous for the storage property, the powder fluidity in a copying machine or in the toner production process, and the granulation and the transferring property of the toner aggregated product by the use thereof for a long period of time.
Furthermore, in the case of forming full color images, the amount of the toners necessary for forming one sheet of image becomes very large in general as compared with the case of forming a black and white image. Thus, the amount of the toner particles remains in every portion such as a development portion, on a photoreceptor, on an intermediate transfer material, etc., become large. Since the remained toner particles sometimes contain a release agent, they are liable to weld to various portions, and these portions become nonfunctional in a short using period of time. Also, because in the case of a full color image formation, the uneven transfer of each color toner largely changes the hue of the color images formed, a higher transferring efficiency is required as compared with the case of a black and white or single color image formation wherein the color concentration only is changed.
To improve the transferring efficiency, various methods of adding an external additive in addition to adding a release agent in the inside of a toner have been investigated. In the case of a silica fine powder generally used as an external additive, it is particularly effective in improving the toner fluidity but it excessively increases the charging property of a negatively charging toner under low temperature and low humidity, and further reduces the charging property by taking in moisture under high temperature and high humidity, there is a problem that a large difference is caused between the charging properties in both cases. As the result thereof, the charging property cannot become the optimum in both under a high temperature and high humidity and under a low temperature and low humidity, and problems such as the inferior regeneration of image density, the formation of a fog on a photosensitive material, the formation of a background fog, staining of the inside of a copying machine are occurred.
For the purpose of solving these problems, a method of selecting a material for particles of an external additive or applying a surface treatment onto these particles of the material is proposed.
However, a sufficient effect has not been obtained by only using these inorganic fine powders. Also, an inorganic oxide added for improving the charging property and the fluidity of the power, such as titania the features that raising of electrostatic charges is higher than silica and also the electrostatic charge distribution becomes sharp owing to, perhaps, the low resistivity of titania. However, in the case of adding titania, a high electrostatic charge cannot be imparted to a toner, and also lowering of the conveying amount thereof, lowering of the density regeneration caused by lowering of electrostatic charges, and a fog on background portions are liable to occur.
Also, when the particle sizes of the external additive are reduced along with small-sizing of a toner, there is a possibility of causing an aggregation, which rather causes a problem of lowering the fluidity or the like of the toner.
For attaining both the improvement of the fluidity and the environmental dependence of electrostatic charging of a toner, it is attempted to add hydrophilic titanium and hydrophobic silica together (Japanese Patent Laid-Open No. S60-136755). By the method, each defect of hydrophilic silica and hydrophobic silica is temporarily restrained but according to their dispersed states, the toner is liable to be influenced by one of the two additives. Particularly, in the case of considering the maintenance thereof, it is difficult to stably control the dispersed structure at the surface of toner and by the stress on a sleeve, each feature of the hydrophilic silica or the hydrophobic titanium is liable to appear. That is, it is difficult to stably control each defect for a long period of time.
Then, a method of adding hydrophilic amorphous titanium oxide to a toner is proposed (Japanese Patent Laid-Open Nos. H5-204183 and H5-72797). Amorphous titanium oxide can be obtained by hydrolyzing a metal alkoxide or a metal halide using a CVD method (e.g., Kagakukogaku Ronbun Shu (Chemical Engineering Papers), Vol. 18, No. 3. 303-307(1992)). However, the titanium oxide obtained by such an electrolysis method can improve both the charging characteristics and the fluidity of a toner but contains many adsorbed water in the inside of the particles thereof and remains on a photoreceptor by itself at transferring. That is, the amorphous titanium oxide has the faults such as the amorphous titanium oxide has a strong sticking force to a photoreceptor, and only the amorphous titanium oxide remains on the photoreceptor without being transferred at transferring of the toner to form unfilled spots on the image formed or scratch the surface of the photoreceptor by the hard titanium oxide at cleaning.
Also, on the other hand, a method of treating the surface of titanium oxide by hydrolyzing a silane compound in an aqueous medium in a method of purifying titanium oxide by a wet method, taking out the titanium oxide in a state of restraining the occurrence of aggregation, and adding the titanium oxide thus treated to a toner is provided (Japanese Patent Laid-Open No. H5-188633).
When the silane compound treatment is applied to titanium oxide by the above-described method, the aggregated particles become less as compared with the hydrophilic method of titanium oxide in the prior art, that is, the improvement of the fluidity of the toner is obtained but, in this case, the charging level and the environmental dependence of the negative-charged toner are almost the same as those of a toner in the prior art, the improvements of the high-negative charging property and the environmental dependence of the toner desired in this invention are insufficient, and further the titanium oxide subjected to such a silane compound treatment gives a bad influence on the charging speed (the admix property of additional toner) and the electrostatic charge distribution.
For solving these problems, in Japanese Patent Laid-Open Nos. H6-95429, H6-102699, H6-266156, etc., it is proposed to use specific binder resins for preventing the embedding of an external additive in toner particles. Also, in Japanese Patent Laid-Open Nos. H6-51561, H6-208242, H6-250442, etc., it is proposed to use specific charge-controlling agents and external additives. However, these effects are insufficient and particularly in a full color development/transfer system of piling 4-color toner images, it is necessary to more precisely control the toner developing amounts, and thus there still remains a problem in the stabilization of the toner charging amounts for a long period of time. Particularly, in the case of using irregular-shaped toners containing a release agent, the charging faculties and the powder characteristics of the toners must be largely improved and thus it is necessary to precisely control the performance of the external additive. However, because a release agent exists in vicinity of the surfaces of toner particles, it is difficult to uniformly add the external additive to the toner particles.